1. Field of the Invention
The present invention relates to a photocoupler, and more particularly to a photocoupler wherein a light emitting device and a photo detector are disposed in a facing relationship to each other.
2. Description of the Related Art
FIG. 1 is a cross-sectional view of a photocoupler in a first example of the prior art. As shown in FIG. 1, a light emitting device 1 and a photo detector 2 are mounted on a single lead frame 3, and a light transmitting silicon resin 7 covers over the light emitting device 1 and the photo detector 2. Then, in order to avoid influence of disturbance light, a light shielding resin 6 encloses (hereinafter referred to as molds) the light emitting device 1, the photo detector 2, the lead frame 3 and the silicon resin 7. For the light shielding resin 6, a resin of a non-black color (cream color or the like) is in most cases employed so as to reflect much of the light emitted from the light emitting device 1. In the first example of the prior art, since the lead frame 3 is constituted by a single frame and the light emitting device 1 and the photo detector 2 are disposed on the same plane, the photocoupler can be produced readily. Since the structure of the photocoupler makes use of reflected light, it is advantageous in that it has a high optical coupling efficiency.
FIG. 2 is a cross-sectional view of a photocoupler in a second example of the prior art. As shown in FIG. 2, a light emitting device 1 is mounted on a lead frame 3a, and a photo detector 2 is mounted on another lead frame 3b. The lead frames 3a and 3b are disposed such that the light emitting device 1 and the photo detector 2 are faced each other. A light transmitting resin 5 molds the light emitting device 1, the photo detector 2, and the lead frames 3a and 3b. A light shielding resin 6 molds the light transmitting resin 5. Also, in order to moderate the stress coming from the light transmitting resin 5, a transparent silicon resin 7 covers over the light emitting device 1. In the second example of the prior art, epoxy resins are generally employed for the light transmitting resin 5 and the light shielding resin 6. By adding filler into the epoxy resins, the thermal expansion coefficients of the light transmitting resin 5 and the light shielding resin 6 can be made equal. Accordingly, even if thermal hysteresis is applied to the product in the production process or upon mounting, the light transmitting resin 5 and the light shielding resin 6 do not exfoliate from each other along their interface, and therefore, the photocoupler of the second example of the prior art is advantageous in that it can resist high voltage for isolation and accordingly it is a high performance electrical isolator.
FIG. 3 is a cross-sectional view of a photocoupler disclosed in Japanese Utility Model Laid-open No. 27067/88. The structure shown in FIG. 3 is substantially the same as that shown in FIG. 1 except for the light transmitting silicon resin 7. Since corresponding devices in FIGS. 1 and 3 are designated by identical reference numerals, overlapping description is omitted herein.
Referring to FIG. 3, in place of the light transmitting silicon resin 7 shown in FIG. 1, a silicon resin 4, in which a light scattering material is contained, covers over a light emitting device 1 and a photo detector 2. Therefore, the structure shown in FIG. 3 has an improved optical coupling efficiency compared with the structure shown in FIG. 1.
FIG. 4 is a cross-sectional view of a photocoupler disclosed in Japanese Utility Model Laid-open No. 78056/89. The structure shown in FIG. 4 is substantially the same as that shown in FIG. 2 except for the silicon resin 7. Since corresponding devices in FIGS. 2 and 4 are designated by identical reference numerals, overlapping description is omitted herein.
In FIG. 4, differing from FIG. 2, a light emitting device 1 is not covered with the silicon resin 7. Then, a cylindrical lens 8 is disposed between the light emitting device 1 and a photo detector 2, which are disposed in a facing relationship to each other, so that a great amount of light emitted from the light remitting device 1 may be condensed into the photo detector 2. Therefore, the structure shown in FIG. 4 has an improved optical coupling efficiency compared with the structure shown in FIG. 2.
FIG. 5 is a cross-sectional view of a photocoupler disclosed in Japanese Patent Laid-open No. 34985/92. In FIG. 5, a light emitting device 1 and a photo detector 2 are disposed in a horizontally facing relationship to each other, and the light emitting device 1 and the photo detector 2 are each covered with a silicon resin 7. A light transmitting resin 5 covers over the area between and around the silicon resin 7 which covers over the light emitting device 1 and the photo detector 2. Then, a light shielding resin 6 molds the light emitting device 1, the photo detector 2, lead frames 3a and 3b, the silicon resin 7 and the light transmitting resin 5.
In FIG. 5, the silicon resin 7 which covers over the light emitting device 1 and the silicon resin 7 which covers over the photo detector 2 serve as convex lenses respectively. Therefore, the structure shown in FIG. 5 has an improved optical coupling efficiency compared with the structure shown in FIG. 2.
With regard to the structures shown in FIGS. 1 and 3, although the optical coupling efficiency is high, since the thermal expansion coefficient it different between the silicon resin 7 or the silicon resin 4 and the light shielding resin 6, the adhesion along the phase boundary between them is weak, so that a dielectric breakdown is very likely to occur there. Therefore, the structures shown in FIGS. 1 and 3 are disadvantageous in that they resist low voltage for isolation and exhibit an insufficient electric isolation performance.
On the other hand, with the structure shown in FIG. 2, although the electric isolation voltage is high, since only the direct light of the light emitted from the light emitting device 1 reaches the photo detector 2, the structure shown in FIG. 2 is disadvantageous in that it decreases optical coupling efficiency.
The structure shown in FIG. 4, directed to solution of the problem described above, has another problem, however, in that it is difficult to attach the cylindrical lens 8 inside the light transmitting resin 5. Also, the structure shown in FIG. 5 has a further problem in that it is difficult to cover over the light emitting device 1 and the photo detector 2 with the silicon resin 7 in the form of lenses. Both of the structures shown in FIGS. 4 and 5 have problems in that the production process is complicated.